Metathesis ring-opening polymerization of cyclic olefin monomers such as norbornene and dicyclopentadiene is typically effected with transition metal catalysts in a mechanism in which olefinic, carbon-carbon double bonds of a monomer are split and transposed to link monomer units with olefinic, carbon-carbon double bonds. It is surmised that transition metal carbenes initiate and propagate such polymerization. See Chapter 2 of K. J. Ivin's Olefin Metathesis, Academic Press, London (1983), for a survey of olefin metathesis catalyst systems. Metathesis catalysts are classified by Ivin as (a) those consisting of an actual metal carbene, (b) those containing an alkyl or allyl group in one of the components from which a carbene ligand can readily be generated and (c) those having neither a preformed carbene nor an alkyl group in any component.
A commonly used catalyst system, e.g. disclosed in U.S. Pat. No. 3,959,234, comprises tungsten, e.g. as tungsten hexachloride, which does not by itself initiate polymerization but which will propagate such polymerization once initiated, and an initiator such as an aluminum, zinc or tin alkyl, e.g. diethylaluminum chloride. Such initiators extremely hazardous; for instance, diethylaluminum chloride is a pyrophoric liquid which ignites spontaneously in air, reacts violently with water and is hazardous to health. Further disadvantages are that such catalyst systems are often difficult to regulate, e.g. to produce thermoplastic polymers of a desirable molecular weight, and produce highly colored polymers, e.g. ranging from dark yellow to pitch black. The preparation of clear polymers is disclosed in U.S. Pat. No. 4,808,635 using a tantalum compound and aluminum alkyl as the metathesis polymerization catalyst system.
An alternative but more costly common catalyst system, disclosed in the 1970 publication of French Patent No. 1,594,934, comprises ruthenium compounds, e.g. ruthenium trichloride hydrate, which produces colored polymers, e.g. pale red to dark yellow. Another disadvantage, as shown in the Examples herein, is that such ruthenium catalysts tend to produce polymers of undesirably high molecular weight, e.g. about 1,000,000 Daltons, and at low yield.